Terminology: say "key derivation operation", not "generator"

Generators are mostly about key derivation (currently: only about key
derivation). "Generator" is not a commonly used term in cryptography.
So favor "derivation" as terminology.

This commit updates the function descriptions.
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index be570c2..01d3069 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -195,7 +195,7 @@
 typedef struct psa_tls12_prf_key_derivation_s
 {
     /* The TLS 1.2 PRF uses the key for each HMAC iteration,
-     * hence we must store it for the lifetime of the generator.
+     * hence we must store it for the lifetime of the operation.
      * This is different from HKDF, where the key is only used
      * in the extraction phase, but not during expansion. */
     unsigned char *key;